Hydraulic fracturing is a process of injecting fluids into a selected oil or gas bearing subsurface earth formation tranversed by a well bore at sufficiently high rates and pressures such that the formation fails in tension and fractures to accept the fluid. In order to hold the fracture open once the fracturing pressure is released a propping agent (proppant) is mixed with the fluid which is injected into the formation.
Hydraulic fracturing increases the flow of fluids from an oil or gas reservoir to a well bore in at least three ways: (1) the overall reservoir area in communication with the well bore is increased, (2) the proppant in the fracture generally has significantly higher permeability than that of the formation, thereby allowing fluids to flow more easily, and (3) the high conductivity channel causes large pressure gradients to be created in the reservoir past the tip of the fracture.
Proppants are generally strong, preferably substantially spherical, particulates that should be able to withstand the high temperatures and pressures and corrosive environments experienced in the subsurface formations surrounding an oil or gas well. Early proppants were formed of material such as glass beads, sand, walnut shells and aluminum pellets. These materials did not have sufficient strength or resistance to corrosion to be successful in many wells, particularly where closure pressures above a few thousand psi were experienced.
U.S. Pat. No. 4,068,718 to Cooke relates to a proppant which Cooke states is formed of "sintered bauxite" that has a specific gravity greater than 3.4. Cooke states that specific gravities above 3.4 are required in order that the proppant have sufficient compressive strength to resist fragmentation under the high stress levels experienced in use. While the proppant described in Cooke's example proved to have sufficient strength to resist crushing, the high specific gravity was undesirable since it required the use of higher viscosity fracturing fluids and resulted in a lower volumetric proppant concentration for a given weight of proppant loading in a fracturing fluid when compared with that achieved by a proppant of lower specific gravity. In general, the higher the volumetric concentration of the proppant in the fracturing fluid the wider the propped fracture will be after the fracturing pressure is released.
U.S. Pat. No. 4,427,068 to Fitzgibbon relates to intermediate strength composite proppants made by mixing calcined diaspore clay, burley clay or flint clay with alumina, "bauxite" or mixtures thereof such that the ratio of alumina to silica in the composite mix is between nine to one and one to one. The powdered starting materials are mixed in an Eirich mixer and while the mixing is in progress sufficient water is added to cause formation of composite spherical pellets from the powdered mixture. Fitzgibbon states that the rate of water addition is not critical. The pellets are dried and then furnaced to sinter the pellets. The sintered pellets have a specific gravity of between 2.7 and 3.4.
U.S. Pat. No. 4,522,731 to Lunghofer relates to an intermediate strength proppant having an alumina content between 40% and 60% which is produced using a spray agglomeration process and which has a density of less than 3.0 gr/cc. In a preferred embodiment Lunghofer produces his proppants from "Eufaula bauxite" which it states is bauxitic-kaolin type material deposited in and around Eufaula, Ala. According to Lunghofer, the Eufaula bauxite preferably contains at least some (above 5%) gibbsite.
U.S. Pat. No. 4,668,645 to Khaund relates to an intermediate strength proppant made from a mined "bauxitic clay" having a specified chemical composition.
The proppants described in the Fitzgibbons, Lunghofer and Khaund patents have specific gravities lower than that of the earlier Cooke proppant and proppants having such lower specific gravities have been used with some success in intermediate depth wells where the stress on the proppant is in the 5,000 to 10,000 psi range. These lower specific gravity proppants, however, all have compressive strengths which are substantially lower than that of high specific gravity materials such as that referred to in the Cooke, et al. patent, which limits the conductivities and permeabilities that are obtained with such proppants, particularly at closure pressures over about 10,000 psi.
The conductivity of a proppant under specific conditions of stress, temperature, corrosive environment and time is the single most important measure of its quality. The conductivity of a packed proppant such as might be deposited in a fracture is defined as the permeability of the proppant pack multiplied by the width of the propped fracture and is usually stated in units of millidarci-feet ("md-ft").
The conductivity of currently available intermediate strength proppants is frequently measured by the tentative API 8 hour procedure, "Tentative Fifth Draft of Recommended Practices For Evaluating Short Term Proppant Pack Conductivity", (Mar. 1987) (hereinafter the "API 8 hour Procedure"), which procedure is hereby incorporated by reference.
Recently a consortium of some twenty-eight organizations involved in various aspects of the fracturing and stimulation business has sponsored research on ways of evaluating and improving stimulation techniques. Stim-Lab, Inc. of Duncan, Okla. acts as the testing arm of the consortium to develop consistent and repeatable testing procedures for proppants including tests for determining their permeability and conductivity. The long term conductivity testing techniques developed by Stim-Lab have been widely accepted in the industry and are described in a publication of the Society of Petroleum Engineers, No. SPE 16900, entitled "An Evaluation of the Effects of Environmental Conditions and Fracturing Fluids on the Long-Term Conductivity of Proppants" by G. S. Penny of Stim-Lab, Inc., which publication is hereby incorporated by reference. It should be understood that any gap in the description in the SPE publication should be filled in by reference to the API 8 hour Procedure. The testing techniques used by the applicants to determine the conductivity of the proppants of the present invention as they are intended to be supplied to a customer (referred to as the "Stim-Lab Technique") are essentially identical to those described in SPE publication No. 16900 using Monel-K 500 shims in the conductivity cells. A single cell was used rather than stacking 4 cells in the manner described in the SPE publication. This however should have no effect on the measured results. The Stim-Lab Technique is considered to yield conductivity measurements that are repeatable to within about 5-10%.